Germ-free animals are animals that have no microorganisms living in or on them. Such animals are raised within germ-free isolators in order to control their exposure to viral, bacterial or parasitic agents. When known strains of bacteria or microbiota are introduced to a germ-free animal, it usually referred to as a gnotobiotic animal, however technically speaking, germ-free animals are also gnotobiotic because the status of their microbial community is known.
Germ-free animals are used in the study of probiotic research and other animal research requiring careful control of outside contaminants that can affect the experiment. They also exhibit defects in the immune system. Because intestinal microorganisms provide additional energy to the animal by breaking down dietary fiber, germ-free animals require more food to keep the same weight than animals that are not germ-free.